How does OpenJDK internally sort the datatypes and why ? It would be great if the specific algorithms can be mentioned
Beginning with version 7, Oracle's Java implementation is using Timsort for object arrays bigger than 10 elements, and Insertion sort for arrays with less than that number of elements. The same considerations apply for both Arrays.sort() and Collections.sort(). In older versions of Java, Merge sort was used instead of Timsort.
Other implementations of the language (other than Oracle's) might use a different sorting algorithm, as this is not mandated by the specification. Quoting Collections' documentation:
The documentation for the polymorphic algorithms contained in this class generally includes a brief description of the implementation. Such descriptions should be regarded as implementation notes, rather than parts of the specification. Implementors should feel free to substitute other algorithms, so long as the specification itself is adhered to. (For example, the algorithm used by sort does not have to be a mergesort, but it does have to be stable.)
For sorting numeric primitives, JDK 7 uses "dual pivot quicksort".